Presently, the use of imported LNG is becoming increasingly important for many countries as the demand for natural gas continues to increase, while domestic production, particularly in the United States and Canada, has been on decline. The imported LNG can make up for a shortfall in domestic production and/or otherwise meet market demand during peak periods, such as during the winter heating season. Such LNG is produced by any of a number of liquefaction methods known in the art, and typically is produced at and imported from a number of remote areas around the world having vast natural gas supply sources, such as the Middle East, West Africa, Trinidad, Australia, and Southeast Asia. After being shipped from such remote locations by specially designed cryogenic tankers, the LNG is typically stored at cryogenic temperatures, until just prior to use, at various locations around the world near locations of high natural gas demand.
It is known that LNG produced from such remote locations in many instances when vaporized does not meet pipeline or other commercial specifications. The resulting natural gas may have an unacceptably high heating value, typically referred to as gross heating value or “GHV”. Various methods have been proposed or used to adjust the GHV of LNG to produce natural gas that will meet pipeline specifications, as is discussed by D. Rogers in “Gas Interchangeability and Its Effects On U.S. Import Plans”, Pipeline & Gas Journal, August 2003 at pages 19-28 and “Long-term Solution Needed To Embrace Imports With Pipeline Gas”, Pipeline & Gas Journal, September 2003, at pages 14-22. For example, such “GHV reduction” or “BTU stabilization” is said by Rogers to be conducted by one or more of the following methods: 1) blending of a high GHV LNG liquid with another LNG liquid having a lower GHV value, such as in the storage tank used to hold LNG prior to sendout; 2) blending of natural gas obtained from a high GHV LNG after vaporization with domestically produced natural gas having a relatively low GHV; 3) injection of an inert gas, such as air or nitrogen, into vaporized LNG prior to its introduction into a pipeline; and 4) stripping heavier hydrocarbons such as ethane, propane, and butane (also known as natural gas liquids or NGLs) from the LNG prior to sendout.
A particular method for NGL removal to lower the GHV of LNG is disclosed by U.S. Pat. No. 6,564,579.
The methods mentioned above generally require significant additional capital costs or have operational problems associated with them. For example, option 1 advanced by Rogers is not very practical as it would either require maintaining a separate inventory of LNG liquids with suitable GHV values, or very careful management of shipments of specific LNG liquids with suitable GHV values for blending with the remaining LNG contained within existing storage tanks. Option 3 would require expensive equipment to conduct the injection into the vaporized LNG, including compressors for raising the pressure up to pipeline pressure, typically as high as 100 bar. Option 4 advanced by Rogers, and the method disclosed in U.S. Pat. No. 6,564,579, would require expensive equipment to remove the desired amount of NGLs.
Conversely, in other parts of the world, such as Japan, there is a desire to increase the GHV of LNG, particularly for LNG having a relatively low GHV produced from sources of natural gas having lower levels of NGLs therein. The GHV can be increased by injection of NGLs or other combustible hydrocarbon materials, such as dimethyl ether, into the LNG such that upon vaporization the resulting natural gas product has an increased GHV.
The LNG is typically stored at low pressure, in liquid form, and at cryogenic temperatures at an import terminal. The LNG is usually pumped to a pressure that is slightly above the pressure of the natural gas distribution pipeline. The high-pressure liquid is then vaporized and sent to the distribution pipeline. The pumping operation typically involves a set of low-pressure pumps located in a storage tank or container connected in series to a set of high-pressure pumps located outside the storage tank.
In many instances in the past, LNG has been vaporized by simply burning a portion of the vaporized LNG to produce the heat to warm up and vaporize the remainder of the LNG and produce natural gas. Various heat exchange systems have been used for this purpose.
As is well known, heat input into the LNG storage tank gradually generates boil-off vapor during storage. Additional vapor generation may occur during filling of the storage container. Vapors may also be obtained from an outside source such as a ship. Ideally, the above-described boil-off vapors are included with the vaporized natural gas sendout into the distribution pipeline. Compressors may be used to boost these vapors to the high operating pressure of the pipeline, which can be as high as 100 bar. However, compressing the vapor to these high pressures requires considerable energy and expensive compressors and related equipment.
U.S. Pat. No. 6,470,706 discloses a system and related apparatus that utilizes cold LNG sendout to condense such boil-off vapors at a low interstage pressure. The teachings of U.S. Pat. No. 6,470,706 are incorporated herein by reference in their entirety. The vapor condensate combines with the liquid sendout and becomes a single phase flow into the high pressure pumps. The combined stream then flows to the vaporizers from the high-pressure pumps. Compressing the boil-off vapor stream to the distribution pipeline pressures requires considerably more energy than boosting the boil-off vapor condensate to the high pressure with a liquid pump.
Other LNG import terminals use systems similar to U.S. Pat. No. 6,470,706 that condense boil-off vapor at low pressure and pump the resulting condensate with the liquid LNG stream flowing to the vaporizer.
It would be desirable to develop a method and system for GHV reduction or BTU stabilization which is more efficient at adjusting the GHV of LNG so that upon vaporization, the resulting natural gas product is interchangeable with domestically produced natural gas or otherwise able to meet set commercial and/or pipeline specifications. It would also be desirable to develop methods and systems that may accomplish the foregoing objectives by relatively simple and low cost modifications to existing systems for vaporization of LNG.